Structure and phase transition of GdBO3

The crystal structure and a phase transition of gadolinium orthoborate, GdB O3, were studied by electron diffraction and X-ray powder diffraction. GdBO 3 crystallizes in the vaterite structure (LT), in a rhombohedral space grou p R32 with the lattice constants of a = 6.6357(2) Angstrom and c = 26.706(1 ) Angstrom . The structure of the LT phase was derived from the hexagonal Y BO3 structure and refined using X-ray powder diffraction data. The structur e consists of tetrahedral polyborate group B3O99- and the gadolinium atoms, located respectively on the 3-fold screw axis and a general position. The material undergoes a first-order phase transition with a large thermal hyst eresis. The high-temperature (HT) phase crystallizes in a calcite related s tructure with the lattice constants of a = 4.1154(2) and c = 8.592(1) Angst rom, which consists of planer triangular berate group BO33-. The observed l arge thermal hysteresis of the phase transition is mainly caused by a struc tural change of the berate group, from B3O99- in the LT phase to BO33- in t he HT phase.